Insufficient weather protection and ultraviolet degradation are problems encountered when using solar reflectors made of a flexible specular silver mirror. When used outdoors, these mirrors must be durable and ultraviolet light (UV) resistant in order to retain their dimensional stability, aesthetic appearance, and specular-reflectance in the visible, ultraviolet, and near infrared wavelengths.
Specular-reflectance is provided in a flexible silver mirror through a silvered composite lamina, having a thin layer of silver vacuum-deposited on the surface of a flexible polymer substrate. Silver is used in some applications because its reflectivity is substantially higher than that of other metals, such as aluminum. To retain specular reflectance over time, some have focused on the application of advanced adhesives and protective layers, coated over the polymer substrate and silver layer, to protect the mirrors from abrasion, weathering, and ultraviolet degradation.
Early techniques used to protect solar mirrors from abrasion, weathering, and ultraviolet degradation were developed with aluminum mirrors. For example, in U.S. Pat. No. 4,307,150, a solar reflector is disclosed wherein an opaque aluminum surface, vacuum-deposited on a flexible polyester support-sheet, is protected from corrosion and weathering with an inter-polymer layer of acrylate or methacrylate copolymers. The support sheet consists of a biaxially oriented polyethylene terephthalate lamina having conventional slip agents, to facilitate winding, and a second polyethylene terephthalate lamina which contains no slip agent.
Silver is higher in specular reflection than aluminum. Thus, the assumption had been to substitute silver for aluminum in the solar reflector described above. However, this approach has been reported, in U.S. Pat. No. 4,645,714, to result in two undesirable phenomena. First, silver is susceptible to corrosion either through the development of pin holes in the acrylate coating or along peripheral portions of the silver-coated film. Second, a thin layer of silver, unlike a thin layer of aluminum, has a spectral window through which ultraviolet (“UV”) light readily passes. A peak transmission of this light is at 320 nm, and sunlight contains ultraviolet light in this wavelength. The transmission of ultraviolet light through the silver layer degrades the underlying polyester substrate causing bubbles in the adhesives, commonly used to adhere the substrate to a rigid support. This degradation and bubbling reduces the aesthetic and specular functional properties of the solar mirror.
Corrosion inhibitors and UV absorbers, incorporated into the adhesives or protective film coatings overlaying a polyester and silver mirror substrate, have been used to retain these functional properties. However, while corrosion inhibitors can reduce corrosion, they can impart an unacceptable color to the mirror, over time, and do not block the ultraviolet light. In contrast, when ultraviolet light absorbers are incorporated into a protective polymer overlay, the rate of polyester support degradation is lessened, but silver corrosion is aggravated. Thus, attempts have been made to isolate the corrosion inhibitor and ultraviolet absorber elements from the mirror's reactive components in order to eliminate these undesirable effects.
In U.S. Pat. No. 4,645,714, a corrosion resistant silver mirror is disclosed wherein a corrosion inhibitor, and an ultraviolet absorber are each incorporated into separate thin overlays of an acrylate inter-polymer paint. The specular reflective mirrors are formed by vacuum-depositing silver over a polyester support film. Ultraviolet degradation of the polyester support, and consequent bubbling of the underlying adhesive, is reduced by incorporating UV absorbers in a second polymer coating that is applied over a first polymer coating, which incorporates a corrosion inhibitor. The first polymer coating is applied directly over the silver reflective surface. U.S. Pat. No. 4,645,714 is explicitly incorporated by reference as though fully set forth herein and to the extent not inconsistent with the disclosure herein.